01 - Preheat the oven to 375°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - Heat a splash of oil in a large skillet over medium heat. Sauté the chopped onion for 3-4 minutes until softened. Add minced garlic and cook for 1 additional minute until fragrant.
03 - In a large mixing bowl, combine cooked chicken, broccoli florets, cooked rice, sautéed onion and garlic, 1½ cups cheddar cheese, sour cream, milk, condensed soup, salt, pepper, and paprika. Mix thoroughly until all ingredients are evenly distributed.
04 - Spread the mixture evenly into the prepared 9x13-inch baking dish, smoothing the top with a spatula.
05 - Sprinkle the remaining ½ cup cheddar cheese evenly over the top of the casserole.
06 - In a small bowl, combine panko breadcrumbs with melted butter. Mix until the breadcrumbs are evenly coated.
07 - Sprinkle the buttered breadcrumbs evenly over the casserole. Bake uncovered for 35-40 minutes until hot and bubbly throughout and the topping is golden brown.
08 - Remove from oven and let the casserole rest for 5-10 minutes before serving to allow the filling to set for easier portioning.